The present invention relates to an image recording apparatus.
A conventional color image recording apparatus such as, e.g., a color electrophotographic printer has multiple processing units containing recording devices arranged in a line as image formation sections. A tandem color image recording apparatus described in Japanese Patent Application Publication 2001-134041 has four processing units, yellow (Y), magenta (M), cyan (C), and black (K), lined up as each type of image forming unit and sequentially copies toner images onto paper electrostatically held and fed on a conveyance belt. In such a color image recording apparatus, the printing speed can be increased because an image in four colors can be printed during a single feeding of the paper. Especially in a processing unit having a miniature LED head or the like as the recording device, the overall size of the apparatus can be reduced by disposing a line head that is mechanically affixed and combined with an image drum unit serving as the image formation section.
However, in the aforementioned conventional color image recording apparatus, due to the insufficient machining accuracy of the unit parts and the like forming the apparatus and the insufficient accuracy of mounting the recording head to the apparatus, it is difficult to prevent displacement between each recording head and image drum unit, and therefore the printing position cannot be held constant in relation to the printing medium. Thus, where a color image made of sequentially overlapping yellow, magenta, cyan, and black toner images in each line is copied, there arises a problem that the printing position is shifted between each color (color shift). A shift of the recording head in the lateral direction crossing the feeding direction of the conveyance belt causes a color shift in the overlapping colors in a main scanning direction. A shift thereof in the longitudinal direction causes a color shift in a secondary scanning direction. In addition, in a case where the mounting position of each line of the recording head is unevenly inclined in the secondary scanning direction, color shift of the overlapping colors occurs in a direction diagonal to the feeding direction. In the conventional color image recording apparatus, when the mounting position of a printing unit is displaced in a main scanning direction, a secondary scanning direction, or a diagonal direction, a displacement arises in the printing location causing a color shift between each of the colors. In order to detect and correct the color shift, the conventional color image recording apparatus prints a stripe pattern overlappingly on another stripe pattern different in color and in separation distance so as to form a patch.
FIG. 17 shows detection patterns 61 through 64 printed on a color image conveyance medium such as, e.g., a recording medium and the conveyance belt, and color shift detection values 0, −1, +1, and −1 respectively corresponding to the detection patterns 61, 62, 63, and 64. When it is detected that the portion of a detection pattern having lower reflective intensity is shifted to right, it is determined that the color recording head is shifted to downstream on the conveyance belt, i.e., a positive direction, with respect to the black recording head. The detection patterns 61, 62, and 63 show cases where there is no color shift of the color recording head with respect to the black recording head. However, in a case of the detection pattern 62, it is falsely determined that the color recording head is shifted to a negative direction since detection is started before the leading edge of the detection pattern 62 arrives at a reflection intensity detection unit 24.
On the other hand, in a case where detection begins after the leading edge of detection pattern 63 arrives at the reflection intensity detection unit 24, a color shift to a positive direction is falsely determined. In such a color shift detection method, an error arises in the color shift detection value because of variation of a time from when exposure begins for the photosensitive body to when the toner latent image arrives at the reflection intensity detection unit 24. To overcome this deficiency, the overall length of the patch having patterns shifted by a certain amount should be equal to or larger than the amount of shift in reading starting position of the reflection intensity detection unit 24 from a reference position.
The patch in FIG. 18 is made from two detection patterns where separation distances are described in the unit of 25.4/1200 mm=1200 dpi. The overall length of the patch is set to approximately 1.5 nm so as to correctly detect the color shift detection value without being affected by the shift in the reading starting position of the reflection intensity detection unit 24.
With such conventional color shift detection methods, it is necessary to configure the length of the patch having patterns shifted by a certain amount to be equal to or larger than the amount of shift in the reading starting position of the reflection intensity detection unit from the reference position, thus resulting in the drawbacks that the overall length of the patch used for the color shift detection process is lengthened.